1. Field of the Invention
The invention relates to a lamp driving circuit with a floating power supply driver and, in particular, to a lamp driving circuit using individual integrated circuits (ICs) prepared in a normal voltage process to control high- and low-voltage driving transistors.
2. Description of Related Art
As shown in FIG. 6, a driving circuit using a current transformer is used to drive a gas discharging lamp 60. The circuit includes a bidirectional switch connected to the DC input side, a current transformer 62, and two power transistors 63, 64 composed of metal oxide semiconductor field effect transistor (MOSFET). The primary winding of the current transformer 62 is connected in series with the lamp 60. Its secondary winding is connected to the two power transistors 63, 64. The lamp 60 is connected in series to an LC circuit composed of an inductor 65 and capacitors 66, 67. The other end of the capacitor 66 is coupled to the primary winding of the current transformer 62.
The above-mentioned circuit provides a pulse current for the circuit to oscillate after the bidirectional switch 61 is closed. Its oscillation frequency is the resonance frequency for the system of the inductor 65 and the capacitor 66. More explicitly, when the power transistor 64 is conductive, the circuit keeps oscillating and uses a high frequency to excite the LC circuit. As a result of the Q resonance of the circuit, the voltage imposed on both ends of the capacitor 67 is amplified sufficiently to trigger the discharge of the lamp 60.
The above-mentioned driving circuit is originally composed of bipolar transistors. After modification, they are replaced by the MOSFET. However, the circuit structure is not suitable for using the MOSFET because it does not have a good wave form.
With reference to FIG. 7, in view of the drawbacks in the previous circuit, U.S. Pat. No. 5,545,955 proposes a driving integrated circuit (IC) 70 suitable for using the MOSFET. The driving IC 70 has a high-voltage output Ho and a low-voltage output Lo coupled to the two power transistors 63, 64, respectively. The driving IC further has an output Vs connecting to the serial connection point of the two transistors 63, 64, the LC circuit, and the gas discharging lamp 60.
According to the working principle of the gas discharging lamp 60, an extremely high driving voltage is required to start the lamp. In fact, inside the driving IC 70, a high-voltage coupling capacitor exists between the high-voltage output Ho and the low-voltage output Lo. It has to be able to withstand hundreds of volts. Due to the existence of the high-voltage coupling capacitor, the material selection, internal device design, and circuit layout of the driving IC 70 have to take into account the high-voltage process. This complicates the manufacturing process and increases the production cost.
The ICs currently used for driving lamps have to be manufactured in a high-voltage process. This is the main reason why the cost and complexity cannot be reduced.